A digitally-edited photo shows the diametrically different views of the Palace Museum, also known as the Forbidden City, before and after Beijing's first-ever air pollution red alert was lifted on Thursday. Photo: CFP

Beijing authorities called off the capital's first-ever air pollution red alert on Thursday, while experts named coal-burning and vehicle exhaust as the main culprits of the heavy smog.

Beijing saw an average PM2.5 concentration of 19 micrograms per cubic meter on Thursday, some 17 times less than yesterday's density. PM2.5 refers to airborne particles smaller than 2.5 microns in diameter, a major air pollutant.

Experts identified the burning of coal and vehicle exhaust as the top two sources of the air pollution that has shrouded the Beijing-Tianjin-Hebei region in recent days, the Legal Daily reported on Thursday.

During the red alert in place Tuesday through Thursday, an odd-even license plate scheme allowed only half of the city's vehicles on roadways, while many construction sites and factories were required to cease operations.

A total of 2,100 companies in Beijing had stopped or limited their production as of Wednesday night, and over 3,500 construction sites had been shut down. Meanwhile, 8,000-plus transport vehicles stopped running on the city's roadways as sanitation vehicles carried out 3,000 dispatches to clean the streets, news site chinanews.com reported.

"Pollutant emissions in Beijing were reduced by around 30 percent on average after the local government took early warning measures to curb air pollution," Cheng Shuiyuan, a professor of environmental and energy engineering at the Beijing University of Technology was quoted as saying by the Legal Daily.

"The density of PM2.5 could have been 10 percent higher without those measures," Cheng said at a conference held by the Ministry of Environmental Protection (MEP).

"Although more data and analysis are needed to determine the red alert's effect on curbing air pollution, the emergency measures have absolutely remitted the process of pollutant accumulation," Wang Gengchen, a research fellow with the Institute of Atmospheric Physics at the Chinese Academy of Sciences, told the Global Times on Thursday.

"The red alert has cut the major sources of pollutants," Wang said.

However, some observers expressed concerns about follow-up efforts, calling for more timely emergency measures to deal with air pollution.

"Emergency measures should be initiated earlier to curb the occurrence of heavy pollution," Wang Zifa, a research fellow at the Chinese Academy of Sciences said at the conference.

The Beijing Municipal Government issued a letter of thanks on Thursday, speaking highly of citizens' cooperation and efforts to deal with the smog.

Unlike Beijing, which has seen a significant improvement in air quality due to the arrival of a cold front, many cities in northern China are still experiencing heavy smog.

The MEP revealed on Thursday that  air quality indexes in some cities in East China's Shandong Province and Central China's Henan Province have exceeded 500.

The capital is expected to be blanketed by smog again on Saturday and Sunday.